The use of electrochemical sensors for the detection of gaseous components and substances has long been known. Such gas sensors generally include at least two electrodes, in the form of one working electrode and one counter electrode. These electrodes find themselves in mutual contact via a conductor or electrolyte.
Such types of gas sensors and gas cells typically involve one side being open to ambient, e.g., by way of a porous membrane. Gas can flow through such a membrane to the electrodes to be electrochemically converted there. The current resulting from the electrochemical reaction ends up being proportional to the quantity of gas. Various arrangements, represented by sulfuric acid or other aqueous electrolytes, have previously been employed as ionic conductors or electrolytes in such gas sensors.
In recent years, the development of gas sensors has tended towards miniaturization. However, the conventionally employed aqueous electrolytes have not lent themselves to miniaturization because of their strongly hygroscopic properties. These hygroscopic properties of conventional electrolytes ensure that, in a dry environment, dehydration of the gas sensor and gas cell is inhibited. However, in high humidity the electrolyte can take on so much water that the gas cell bursts as a result, and the electrolyte leaks out. To prevent such leakage of electrolytes, it becomes necessary to increase the inner volume of the gas cell to 5 to 7 times the electrolyte fill volume. However, this prevents any meaningful miniaturization of gas cells and gas sensors.
By way of an alternative, electrolytes have manifested as ionic liquids in recent years. Ionic liquids have proven to be unique solvents showing solubility, miscibility and other physiochemical properties (e.g., non-volatile properties) over a broad range.
Ionic liquids are, per definition, liquid salts with a melting point under 100° C. The salt structure of ionic liquids requires a correspondingly negligible vapor pressure. Many ionic liquids are very stable chemically and electrochemically, and feature high conductivity. Some ionic liquids, especially those with hydrophobic cations and/or anions, exhibit relatively low water absorption. At the same time, other ionic liquids show water absorption similarly to an aqueous salt solution. In contrast to aqueous salt solutions, however, these ionic liquids still show an electrical conductivity even at extremely low humidities, while, because of water evaporation, this is not the case for aqueous salts such as LiCl solutions.
During the past decade, the inclusion of ionic liquids in gas sensors was concertedly investigated. As such, the use of gas sensors with ionic liquids used as electrolytes, for the detection of acid gases such as sulfur dioxide or carbon dioxide, has been described (WO 2008/110830 A1, WO 2010/063626 A1).
Different ionic salts with different properties and potential applicability in electrochemical gas sensors were intensively explored. Thus, by way of example, ionic liquids were used based on given cation classes in combination with halide, sulfate, sulfonate, borate, phosphate, antimonate, amide, imide anions. Typical cations are substituted imidazolium ions, pyridinium ions, pyrrolidinium ions, phosphonium ions, ammonium ions and guanidinium ions (DE 10 2005 020 719 B3).
Electrochemical sensors for the detection of ammonia are typically based upon direct oxidation of the gaseous ammonia in the context of molecular nitrogen formation and electron release. However, such sensors exhibit reduced stability, which especially is brought about by exposing the sensor to the ammonia gas for longer time periods.
Another potentiometric measurement principle in ammonia sensors is premised on direct or indirect pH measurement. In such sensors, the ammonia under detection is converted into ammonium ions and hydroxide ions via the water of the electrolyte being employed. This approach is followed, e.g., in EP 1 183 528 B1, in which a sensor for the detection of ammonia and amines is described, incorporating an electrolyte which contains oxidizable Mn2+ and a suitable organic solvent. The measurement electrode includes a surface with a catalyst which, in the presence of the gas being measured, catalyzes the oxidation of Mn2+ into Mn4+. The measurement principle realized here follows this reaction scheme:NH3+H2O→NH4++OH−  (I)Mn2++2H2O→MnO2+4H++2e−  (II)
The oxidation reaction of the Mn2+ is possible because of the pH shift following reaction (I). This pH shift also shifts the redox potential of the Mn2+ oxidation. It has been shown here to be disadvantageous that the MnO2 precipitates from the electrolyte and blocks the measuring electrode and the gas inlet membrane, whereby gas input is reduced significantly. Therefore, such sensors exhibit no type of adequate long-term stability.
Also, the one step that determines reaction speed here is the introduction of equilibrium between electrolyte and gas space. Thus, in addition to low stability, this type of measurement system also has the disadvantage of a relatively long response time with the type of ammonia sensor at hand. As such, a different measurement principle is embraced in DE 38 41 622 C2, whereby the provision of gas sensors for ammonia with relatively short response times is facilitated. In DE 38 41 622 C2, a soluble, non-oxidizable substance is added to the electrolyte, which undergoes a reaction with ammonia during formation of an oxidizable product. In turn, this oxidizable product can be converted into chemically and electrochemically inert byproducts via electrochemical oxidation. Thus, the actual electrochemical reaction is preceded by an equilibrium reaction of ammonia with a non-oxidizable substance, which itself leads to a complete conversion of the ammonia into an easily oxidizable product. Such easily oxidizable products are then oxidized at the measurement electrode. Tris(hydroxymethyl)aminomethanhydrochlorid (Tris-HCl) has proven to be especially suitable for this purpose.
In an acid-base reaction that precedes the actual detection reaction, ammonia diffusing into the gas sensor reacts with the Tris-HCl into an ammonium ion and the corresponding organic amine of Tris-HCl. Further, the organic amine is oxidized electrochemically at the measurement electrode, such that the electrons released at that point contribute to the measurement cell current. The organic amines oxidized at the electrode thence break up into additional reaction products. This measurement principle, by way of example, is set forth in the equations shown in FIG. 1.
And yet, a disadvantage with the described measurement system for ammonia is that the Tris-HCl being used is introduced into a liquid electrolyte. As described hereinabove, the use of aqueous electrolytes does provide a hindrance to the miniaturization of gas sensors as well as performance limitations at low humidity conditions.